Method of producing color filters

ABSTRACT

A method of producing color filters comprising: 
     a step for forming a photosensitive film by coating, on a rugged substrate, a mixture of a light-absorbing material which absorbs light and which can be removed with a solvent and a photosensitive material; 
     a step for irradiating light onto the photosensitive film through a predetermined pattern; 
     a step for removing dissolvable portions of the photosensitive film and the light-absorbing material from the photosensitive film which remains on the substrate; and 
     a step for dyeing the photosensitive film into any desired color.

BACKGROUND OF THE INVENTION

The present invention relates to a method of producing color filters byforming a color filter array directly on a rugged substrate consisting,for example, of solid state imagers.

According to the conventional methods of producing color filters, aphotosensitive material is coated on a flat substrate made of, forexample, a glass, the photosensitive material is exposed through a maskof a predetermined pattern, the development is effected to formpatterns, and the patterns are dyed with dyes. The thus obtainedsubstrate with filter has been used being placed on the solid stateimagers. With the above-mentioned method, however, when the color filterof fine patterns is placed on small solid state imagers, the thicknessof the substrate or the deviation in position when the color filter isbeing placed makes it difficult to increase the resolution of the cameratube.

If it is attempted to directly form the color filter on the substrateconsisting of solid state imagers having rugged surfaces, theultraviolet rays which are vertically incident upon the substrate duringthe step of exposure are irregularly reflected through the rugged orstepped portions; the ultraviolet rays proceed in parallel with thesubstrate up to the inner side of the patterns that are to interrupt theultraviolet rays, making it difficult to properly shape the patterns.This state is illustrated below with reference to FIG. 1. Ultravioletrays 2 transmitted through a transparent glass portion 6 of an exposuremask 1 are reflected by a tilted surface 3 of a substrate 7 locatedbeneath the mask 1, and are introduced into a photosensitive resin 5beneath a opaque portion 4 of the mask 1, preventing the photosensitiveresin from being shaped into a predetermined pattern. Because of thesereasons, it is difficult to form color filters of fine patterns directlyon the solid state imagers.

LIST OF PRIOR ART (37 CFR 1.56(a))

The following references are cited to show the state of the art.

Japanese Published Examined Patent Application No. 51-37562

Japanese Published Examined Patent Application No. 52-17375

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method of producingcolor filters of fine patterns on a substrate having rugged surfaces.

Another object of the present invention is to provide a method ofproducing color filters comprising;

a step for forming a photosensitive film by coating, on a substrate, amixture of a light-absorbing material which absorbs light and which canbe removed with a solvent and a photosensitive material;

a step for irradiating light onto the photosensitive film through apredetermined pattern;

a step for removing dissolvable portions of the photosensitive film andthe light-absorbing material from the photosensitive film which remainson the substrate; and

a step for dyeing the photosensitive film into any desired color.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for illustrating the conventional technique;

FIGS. 2, 3, 4 and 5 are views for illustrating the present invention;and

FIGS. 6, 7, 8, 9 and 10 are cross-sectional views of an element forillustrating an embodiment according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The feature of the present invention is to effect the exposure anddevelopment using a photosensitive resin which contains alight-absorbing material. According to the conventional art, thephotosensitive resins are usually blended with a sensitizer having goodtransmittance. In the present invention, on the other hand, problemsinherent in the conventional art are solved by adding a light-absorbingmaterial.

According to the method of the present invention, the light which isreflected toward the photosensitive resin is absorbed or weakened by thelight-absorbing material, and is so drastically attenuated that thephotosensitive resin is very less polymerized, making it possible toaccurately form patterns which are substantially corresponding to anexposure mask. Examples of the light-absorbing material include directdyes, acid dyes, basic dyes, mordant dyes, reactive dyes, and the like.The light-absorbing material, however, must be capable of absorbing partof the light of wavelengths ranging from 350 to 450 nm. Furthermore, thelight-absorbing material must be capable of being dissolved in thephotosensitive resin and must be capable of being removed after patternshave been formed so that the spectral transmittance of the filter is notdeteriorated.

FIG. 2 shows the transmittance of ultraviolet ray of 365 nm when aTartrazine (trade name,3-carboxy-1-(p-sulfophenyl)-4-(p-sulfophenylazo)-5-hydroxy pyrazole)which is an acid dye having absorption characteristics in theultraviolet regions, is added to a negative-working photoresist(photoresist of the type of gelatin-ammonium dichromate) which isusually used as a water-soluble photoresist. In this case, the thicknessof the film is 1 μm. As will be obvious from FIG. 2, the transmittanceof ultraviolet rays is drastically reduced by the addition of a verysmall amount of Tartrazine which is a light-absorbing material to thegelatin. The amount of Tartrazine is expressed by percent by weight withrespect to the gelatin. FIG. 3 shows the transmittance of 435 nm.

The same effects are also observed in this case.

FIG. 4 shows spectral characteristics of a film prepared by addingTartrazine to the photoresist of the type of gelatin-ammoniumdichromate, in which curves 41, 42, 43 and 44 represent transmittanceswhen Tartrazine is added in an amount of 1% weight (with respect togelatin, the same holds true hereinafter), 3% by weight, 5% by weight,and 10% by weight, respectively.

The light-absorbing material should be added in such an amount that thetransmittance of ultraviolet rays irradiating the photosensitive resinis smaller than 80% but is larger than 20%, and more preferably in suchan amount that the transmittance is smaller than 75% but is larger than40%. The operability becomes poor when the transmittance is smaller than20%. Therefore, the amount of the light-absorbing material actuallyadded will vary depending upon the wavelengths of the ultraviolet raysand the kind of light-absorbing materials. For instance, when thephotosensitive resin is to be irradiated with ray of a wavelength of 435nm, the Tartrazine should be added to the photosensitive resinpreferably in amount of 0.5% by weight (with respect to gelatin) to 10%by weight, and more preferably in an amount of 1% by weight to 5% byweight.

In addition to Tartrazine, examples of the light-absorbing materialinclude the following:

As acid dyes;

Fluorescein sodium salt (Color Index No. (hereinafter abbreviated asC.I. No.) 45350);

Orange G (C.I. No. 16230, sodium 7-hydroxy-8-phenylazo-1,3-naphthalenedisulfonate);

Orange I (C.I. No. 14600, 4-(p-sulfophenylazo)-1-naphthol);

Orange II (C.I. No. 15510);

Acid Light Yellow 2G (C.I. No. 18965);

Kayanol Yellow N5G (trade name, produced by Nippon Kayaku Co., Ltd.);

Kayanol Yellow NFG (trade name, produced by Nippon Kayaku Co., Ltd.);and

Suminol Fast Yellow G (produced by Sumitomo Chemical Co., Ltd.).

As basic dyes;

Acridine Yellow (C.I. No. 46025, 2,7-dimethyl-3,6-diaminoacridinehydrochloride);

Chrysoidine (C.I. No. 11270, 2,4-diaminoazobenzene);

Acridine Orange (C.I. No. 46005, zinc double chloride of3,6-bis(N,N-dimethylamino)-acridine hydrochloride); and

Yellow 3G-N (trade name of Mitsubishi Chemical Co.).

As direct dyes;

Direct Pure Yellow 5G (C.I. No. 13920,p-(6-methyl-7-sulfobenzo-2-thiazolyl)-phenylazoacetoacetanilide);

Brilliant Yellow (C.I. No. 24890); and

Chrysophenine G (C.I. No. 24895).

As mordant dyes;

Chrome Yellow (C.I. No. 14095, sodium2-(3'-carboxy-4'-hydroxyphenylazo)-6-naphthalene sulfonate).

As reactive dyes;

Brilliant Yellow 7-GL (trade name of Mitsubishi Chemical Co.);

Brilliant Yellow GL (trade name of Mitsubishi Chemical Co.);

Mikacion Yellow GRS (trade name of Mitsubishi Chemical Co.); and

Drimarene Yellow Z-RL (trade name of Sandoz Co.).

The above-mentioned dyes are all soluble in water.

Even when these light-absorbing materials are added, the photosensitiveresin does not almost lose adhesiveness or resistance against chemicals.

The photosensitive material should desirably be soluble in water. Thepresent invention employs a widely known photosensitive materialcomposed of a high molecular substance such as gelatin, polyvinylalcohol, or the like, and a crosslinking agent such as ammoniumdichromate, bisazide, or the like.

Below are mentioned the results when the resin coated on a solid stateimager having rugged surface composed of aluminum is exposed to thelight through mask. FIG. 5 illustrates a relation between the widthwhich is sensitized by the light that is reflected from beneath theexposure mask and the quantity of exposure. A curve a represents thecase when the light-absorbing agent is not added, and a curve brepresents the case when Tartrazine which is a light-absorbing agent isadded in an amount of 3% with respect to gelatin. As will be obviousfrom FIG. 5, under the equivalent exposure (to equivalently sensitizethe photoresist, the effect of the light-absorbing agent must be takeninto consideration because it usually works to reduce the sensitivity),the region sensitized in the opaque portion of the mask pattern due tothe reflected light is very narrowed when the light-absorbing agent isadded, and the dimensional precision of pattern is very littledeteriorated by the light reflected by the tilted surfaces. The quantityof exposure in FIG. 5 has the following meaning. The photocrosslinkingreaction in the negative-working photoresist proceeds with exposure. Thephotoresist becomes insoluble in the developing liquid, so that a filmremains. The thickness of the film increases with the increase in thequantity of exposure. As the quantity of exposure exceeds apredetermined value, the thickness becomes nearly in agreement with thethickness when the photoresist was coated. A minimum quantity ofexposure at which the coating of photoresist is all sensitized to form afilm is generally referred to as a minimum required exposure quantityand is used as a unit of exposure quantity. The exposure of FIG. 5 isthus defined.

The effect of the light-absorbing agent is illustrated below from thestandpoint of actually handling the photo-sensitive resin. Thenegative-working photoresist undergoes the crosslinking and becomesinsoluble in the developing liquid. When the degree of crosslinking issmall, however, the film is extremely swollen or is peeled off. Toprevent the swelling or peeling during the step of developing,therefore, the exposure is usually excessively performed. The quantityof exposure should usually be two to three folds. With reference to theeffects of light-absorbing agent of FIG. 5 in which the quantity ofexposure is three folds, the width sensitized by the reflected light isreduced to about one-fourth contributing to the remarkable improvementin the precision. Thus, it is made possible to form fine patterns.

The light-absorbing material is removed by solvent from thephotosensitive film simultaneously with the development of thephotosensitive film, or before or after the development of thephotosensitive film. Namely, the light-absorbing material may be removedbefore, simultaneously with, or after, the development provided theexposure has been completed. When the photosensitive resin of the typeof gelatin is used, the development is usually carried out in hot water.Therefore, when the light-absorbing material which can be removed withwater is added to the photosensitive resin of the type of gelatin, it isallowed to remove the light-absorbing material simultaneously with thedevelopment.

The present invention further provides a method of removing thelight-absorbing material with weakly alkaline water. When it isattempted to remove the light-absorbing material immediately after ithas been mixed into the photosensitive film, the light-absorbingmaterial can be completely removed. However, when the light-absorbingmaterial is left to stand in the photosensitive film for a while, itwill not be completely removed causing the photosensitive film to becolored to a slight degree. The color adversely affects the spectraltransmittance of the color filter. To practically produce fine colorfilters, the operation such as focusing must be precisely carried outrequiring sufficient periods of time. The film, however, will be coloredas mentioned above when extended periods of time are lapsed. It istherefore recommended to remove the light-absorbing material from thephotosensitive film using a weakly alkaline aqueous solution having a pHvalue of preferably from 7.1 to 12.0, and more preferably having a pHvalue of from 9.2 to 12.0. The alkaline aqueous solution having a pHvalue in excess of 12 causes the film to be peeled off. The pH valuetherefore should not be greater than 12.

The film will be easily peeled off when the temperature of the weaklyalkaline aqueous solution is high. On the other hand, the development isusually carried out in hot water. It is not, therefore, desirable toeffect the developing and the removal of light-absorbing materialsimultaneously using the developing solution as a hot and weaklyalkaline aqueous solution. It is recommended to remove thelight-absorbing material with weakly alkaline water of room temperatureafter the developing has been effected.

Examples of the weakly alkaline aqueous solution will include ammoniawater, aqueous solution of amine, aqueous solution of alkali hydroxide,aqueous solution of carbonates, aqueous solution of an alkali metal saltof weak acid. For example, aqueous solutions are employed such as oftetramethyl ammonium hydroxide, pyridine, diethanol amine, sodiumacetate, sodium carbonate, sodium hydrogen carbonate, sodium hydroxide,and the like.

The photosensitive film from which is removed the light-absorbingmaterial exhibits quite the same absorption spectrum as that of thephotosensitive film which does not contain light-absorbing material butwhich is exposed and developed.

The photosensitive film from which has been removed the light-absorbingmaterial is dyed by a conventional method, for example, by a methoddisclosed in Japanese Patent Publication No. 52-17375, to obtain a colorfilter having fine patterns.

Japanese Patent Publication No. 51-37562 discloses to effect theexposure and development by using a photo-sensitive film which includesa light-absorbing material. According to this method which employs anoil-soluble dye, however, it is very difficult to decolor the dyeswithout affecting the photosensitive film. In the method disclosed inthe above Japanese Patent Publication No. 51-37562, the substrate afterdevelopment is subjected to the etching, and then the remainingphotosensitive film is removed. If the film containing a light-absorbingmaterial which is difficult to decolor is dyed, two colors are mixedtogether making it impossible to obtain a color filter having desiredspectral characteristics.

The present invention features that use of the above-mentionedlight-absorbing material enables the color to be completely removed, andparticularly that use of a weakly alkaline aqueous solution enables thecolor to be completely removed even though such color may be difficultto be completely removed under ordinary conditions. Owing to the methodof the present invention, it is allowed to make a color filter which isprecisely dyed to a desired color having precise and fine patternscorresponding to desired patterns.

The invention is illustrated below in further detail with reference toembodiments.

EXAMPLE 1

Tartrazine which is a light-absorbing agent is added in an amount of 3%with respect to gelatin to a water-soluble photoresist of the type ofgelatin-ammonium dichromate. The mixture is then uniformly coated on asolid-state imager 8 having rugged surfaces to form a film 9 as shown inFIG. 6. The film 9 is dried and immediately thereafter, ray 2 having awavelength of 435 nm is irradiated through a mask 11 onto the portionswhich exhibit a first color as shown in FIG. 6. Developing is theneffected in hot water to remove unexposed portions of the photoresist.In this case, the Tartrazine is eluted out, and the gelatin filmexhibits spectral transmittance characteristics which are the same asthose of the gelatin films without admixed with Tartrazine. Then, theportions corresponding to the first color 12 are dyed with a dye havingpredetermined spectral characteristics thereby to prepare a colorfilter. Then, with reference to FIG. 8, a transparent intermediate layer13 is coated to prevent colors from being mixed together. To preparecolor filters having second and further colors, the operation asmentioned with reference to the formation of the first color should berepeated as illustrated in FIGS. 9 and 10.

EXAMPLE 2

The photosensitive film prepared in the same manner as in Example 1 isleft to stand for a predetermined period of time, and is then exposedand developed. Since the photosensitive film is slightly colored, thefilm is immersed in an ammonia water of 0.1% for 30 seconds, washed withwater and is dried. The Tartrazine is completely removed, and thephotosensitive film exhibits spectral transmittance characteristicswhich are the same as those of the photosensitive films which do notcontain Tartrazine and which are exposed and developed. The film is thendyed and is coated with an intermediate layer like Example 1. Colorfilters of the second and third color are then formed by repeating thesame procedure.

The following experiment helps confirm that the light-absorbing materialis also removed and color is extinguished when a weakly alkaline aqueoussolution is used instead of ammonia water. Namely, a film is preparedfrom an aqueous solution of photosensitive resin composed of gelatin,ammonium dichromate (5% by weight with respect to gelatin) andTartrazine (1.5% by weight with respect to gelatin), left to stand for100 minutes, irradiated with light (70,000 luxes for 11 seconds), and isdeveloped (in hot water maintained at 40° C. for 2 minutes). The filmexhibits a transmittance of about 70% at around 415 nm, indicating thatit is colored. When immersed in an aqueous solution of tetramethylammonium hydroxide (pH 12), in an aqueous solution of pyridine (pH 9.2),and in an aqueous solution of diethanol amine (pH 11.2), respectively,the color of the gelatin film is extinguished. Namely, the film exhibitsthe same spectral transmittance characteristics which are the same asthose of the photosensitive resin which does not contain Tartrazine butwhich is exposed and developed.

According to the present invention as mentioned in the foregoing, thelight transmittance of the photo-sensitive resin is decreased by addinga light-absorbing agent to the photosensitive resin, thereby to formfine patterns on the surface of a substrate which has a rugged surfaceand good reflection factor. Although the aforementioned embodiments haveemployed a photoresist of the type of gelatin-ammonium dichromate as thephotosensitive resin, it is of course allowable to use a water-solublephotosensitive resin. Furthermore, the light-absorbing agent needs notbe limited to the Tartrazine only, but those which absorb light whilethey are contained in the photosensitive resin and which can be removedby the developing after the patterns have been formed, can also beemployed for the present invention.

What is claimed is:
 1. A method of producing color filters comprising:astep for forming a photosensitive film by coating, on a substrate, amixture which contains a water-soluble, negative-working photo-sensitivematerial and a water-soluble, light-absorbing material which absorbslight, which can be removed with a solvent and which can be dissolved inthe photosensitive material; a step for irradiating light onto thephotosensitive film through a predetermined pattern; a step for removingdissolvable portions of the photosensitive film and the light-absorbingmaterial from the photosensitive film which remains on the substrate;and a step for dyeing the photosensitive film into any desired color. 2.A method of producing color filters according to claim 1, wherein saidlight-absorbing material is mixed to said photosensitive material insuch an amount that the transmittance of light is smaller than 80% butis greater than 20%.
 3. A method of producing color filters according toclaim 1 or 2, wherein said light-absorbing material is comprised of atleast one dye selected from the group consisting of direct dye, aciddye, basic dye, mordant dye and reactive dye.
 4. A method of producingcolor filters according to claim 1, wherein said water-solublenegative-working photosensitive material is a gelatin-dichromateadmixture.
 5. A method of producing color filters according to claim 1,wherein the removal of dissolvable portions of said photosensitivematerial and the removal of said light-absorbing material aresimultaneously carried out in hot water.
 6. A method of producing colorfilters according to claim 1 or 2, wherein said substrate is asolid-state imager.
 7. A method of producing color filters according toclaim 1, wherein the light-absorbing material is removed with a weaklyalkaline aqueous solution.
 8. A method of producing color filtersaccording to claim 7, wherein said light-absorbing material is removedafter the dissolvable portions of said photosensitive film have beenremoved.
 9. A method of producing color filters according to claim 7,wherein said weakly alkaline aqueous solution has a pH value over arange of from 7.1 to 12.0.
 10. A method of producing color filtersaccording to claim 7 or 8, wherein said light-absorbing material ismixed to said photosensitive material in such an amount that thetransmittance of light is smaller than 80% but is greater than 20%. 11.A method of producing color filters according to any one of claims 7 to9, wherein said light-absorbing material is comprised of at least onedye selected from the group consisting of direct dye, acid dye, basicdye, mordant dye and reactive dye.
 12. A method of producing colorfilters according to any one of claim 7 to 9, wherein said water-solublenegative-working photosensitive material comprises a gelatin-dichromateadmixture.
 13. A method of producing color filters according to any oneof claims 7 to 9, wherein said substrate is a solid-state imager.
 14. Amethod of producing color filters according to claim 1, wherein saidlight-absorbing material is a material capable of absorbing a part oflight having wavelengths ranging from 350 to 450 nm.
 15. A method ofproducing color filters according to claim 1, wherein saidlight-absorbing material comprises at least one dye selected from thegroup consisting of direct dye, acid dye, basic dye, mordant dye andreactive dye, and said photosensitive material comprises a photoresistcontaining a mixture of a water-soluble polymer and a crosslinking agentfor said polymer.
 16. A method of producing color filters according toclaim 15, wherein said water-soluble polymer comprises gelatin orpolyvinyl alcohol and the crosslinking agent comprises ammoniumdichromate or bisazide.